Automatic frequency control



July 25, 1944. c. w. EAR 2,354,510

AUTOMATIC FREQUENCY CONTROL Filed July 17, 1942 OSCILLATOR 0 1 cm Lsaaaa'winm l VOLTA5 C V F V FREQUENCY com-non.

F1 fig. Z. F f) FILTER -r MODULATOR MODULA on MI DELAY nnwonx rnsouzucvDISCRIMINATOR cow ring. P 1 F OSCILLATOR comnov. VOLTAGE CV man 1,3JNTERMEDIATE rnzquzncy 3 rasouzucv AMPLIFIER mxsn AMPLIFIER DETE TORH1 H7 I [F4 DET MODULATOR MODULATOR F1 MI- fl f) v nun nun 'DISGRIMINATOR I mrj V DISCRIMINATOVR F O R g v PC .6 //VI. /Wfl/POSCILLATOR 1W I CONTROL RECTIFIER (9% C. W Efl/FP Patented July 25, 1944UNITED STATES I {PATENT 2,354,510 AUTOMATIC FREQUENCY CONTROL.

Charles William EarpQLondori, England, assignor to InternationalStandard Electric Corporation, NewYork, N. Y.

Application July 17, 1942, Serial No. 451,292 In Great Britain July 11,1941 7 Claims. (012 250-40) whichit is set, the value of sa d directcurrent voltage determining the rate of changeofire-,5 quency of saidoscillator and bringing the fre ,quency thereof to such a value as tomaintain said, Q intermediate frequency constant,

In carrying out the invention theoscillator 1 such that it remainsstable in frequency at any 71 frequency to which it is set and Withinthe band,

width of the control circuits but 'is -readi1y iis -v l narrower aspectof the invention,

can oscillate on any of a wide range of frequencies comprises-anoscillator and a frequency dis-- criminator or countingarrangementadapted to produce a direct current voltage in the output thereofdepending'upon the frequency of the in put fed to the said discriminatoror-counting arrangement, and means for feeding said output I voltage tosaid oscillator to -maintainthe frequency generated by the latter.

According to another narrower aspect of the invention, a circuitarrangement for the stabilisation of an oscillator at any frequency towhich it is set comprises .a frequency discriminator'or countingarrangement fed from the oscillator and adapted to produce a directcurrent voltage in turbed in frequency by applicationof-an addi: tionalcontrol voltage from an ext'ernaleource;

The invention will be better understood fromthe. following description.takenin conjunction ,lfi with the accompanying drawing in which Fig. 1shows in block;schematic a ifrequency-j control arrangementaccordingtothe invention applied to an oscillation generator; V a, Fig.2 shows in block schematic the. invention applied to another andpreferredformof oscillation generator; and l 1 Fig. 3 shows in blockschematic the invention incorporated in a superheterodyne' receiyen:Referring to Fig. 1, the oscillator O .feeds.a;

; ;.frequency discriminator D, which provides -an--in-.

the output thereof depending upon the frequency of the input and meansfor feeding said output direct current voltage to said oscillator insuch manner that the value of said output voltage maintains thefrequency of said oscillator, the said oscillator and discriminatorbeing so chosen that their characteristics showing the relationship:between frequency and controlling voltage or frequency and outputvoltage respectively, are substantially linear,- and have equal slopes,in the vicinity of the required stabilised frequencies.

,The' invention is particularly suitable for application totheoscillator of a superheterodyne radio receiver to supply thefrequency changes of the oscillator where the received signal drifts infrequency so as to maintain the resulting intermediate frequencysubstantially constant.

According to a feature of the invention therefore, in a superheterodynereceiver arrangements are provided for eliminating the effect due tofrequency drift of the received waves. These arrangements comprise afrequency discriminator or counting arrangement fed from the output ofthe intermediate frequency stage and adapted to produce a direct currentvoltage in the output thereof depending upon the input frequency, andmeans for feeding said output current voltage to said oscillatorstabilised at any frequency to put voltage to the frequency control tubeFV. 1 in turn coupled back to the oscillatoreO}. The'e fre- 1. quencycontrol tube operates on well known prin-"g ciples, and may depend,forexample,-.-on the Now, it is arranged that for 'agiven displacementin frequency-of the-oscillator O, the discriminator sensitivity is; suchthat-its-l change of output voltage is exactly that, which is requiredto produce that same change "i'n'fre quency by application of thevoltage to 'the fre=- quency control tube. If, now, the discriminatorcharacteristic (input frequency versus output voltage) exactly matchesthe frequency control tube-oscillator characteristic, .(inputv-voltage"to versus output frequency of 0*) the oscillator.

does not tend to drift from 'an'y'frequency to which it has been set. t

A small additional frequency control :voltage, applied from someexternal source represented at CV will now cause the oscillatorfrequency to;

increase or decrease according to the sense of.

p in the path Ml to M2.

the frequency applied to the discriminator.

nected directly to a modulator M2, and via a I phase controllingthermionic tube PC to modulator Ml, to which a frequency band of meanfrequency f is also passed as hereafter explained. Filter Fl selectsfrom the output of MI a band of frequencies, of mean frequency (Ff) (orF+f) which is passed to M2.

control tube PC so as to shift the frequency in the opposite direction.

If, now, the D. C. voltage from the discriminator D, as applied to thecontrol tube PC, is supplemented by an additional D. C. frequencycontrol voltage from an external source CV, the frequency of theoscillations speeds up or slows down, according to the sense of thecontrol voltage, until the control voltageis removed.

The frequency for controlling the oscillation circuit may be obtainedfrom the output of either filter. Likewise the output of either filtermay The function of the delay network DL'is to v comprise a voltagegenerated in response to the provide a linear phase shift of 0 radiansper A 'cy cles change in the frequency f. If the freproduced byjthedelay network .DL will be ad'- vanced 'jby afradians. Similarlyanyjother fre quencyijchange between +,A ','and A willproduce acorresponding phase'shift between +0 and -0.

If the amplification around the circuit is sufficiently great,oscillations are set up, and these oscillations must be 'of'meanfrequency ,f, in order to pass throughv the filter F2, and also offrequency Fif to pass through the filter Fl. The delay network isdesigned to give a linear distortion over a frequency band of ,f- -A. Ifa certain'frequencyj is produced when the frequency F, applied to themodulator Ml, has a definite'phase relationship to the output of theoscillator 0' applied tothe modulator M2, and if a change in phaserelationship occurs, the correct phase relationship around the loop hasnow been upsetand' the frequencyf will rise or fall to accommodateitself to the new conditions.

For instance, ifthe phase of the frequency'F injectd'jfiiito"thmodulatorMl is advanced by the 'phasebontrol tube'PC 0 radians withrespect'to tlie sani'freq'uencyF from the oscillator 0 'directlyinjected into the modulator M2, oscillations can only be maintained inthe loop if the'frequency ,f is increased to f+A and the frequencyFif-is increased to F:(f+A). Oscillations at these new frequenciesarequite stable. FilterFZ selects from the output of M2 the frequencyband of 'mean frequency j, which is passed to Ml. At some point in theloop path MI'M2MI is inserted a delay network DL or alternativelytherequired delay may be produced in 'any'of the components of the circuit,for instance,-thc filters.

The circuit as described up to this point producesoscillation'offrequency f in the path M2 to Ml,and'oscillation of frequency F-f orF+,f Now, as explained above and in the said specification ofapplication 460,578, a phase shift produced by the phase controltube PCproduces a corresponding'change in the frequency The frequency f is nowfed from as constant of the circuit between the discriminafilter..F2-to' a discriminator D, or frequency counter, the ouput voltage of whichis applied to the phase controltube PC and exactly matches thephase'control voltage necessary to maintain With the discriminatorconnected in this manner the frequency fwill be held constant, since anychange thereof will produce a voltage in the output'of the discriminatorD which will affect the ceiver.

be used for any desired purpose, for example, as

the local oscillation of a superheterodyne re- As shown, the filter Flis shown with an outputcircuit for this purpose.

In either of the arrangements shown in Figs. 1 and 2 the source ofcontrol voltage CV may deviation of the intermediate frequency fromnormal in a superheterodyne receiver. Such a receiver incorporating theinvention is shown in Fig. 3. In Fig. 3, HF. is the high frequencyamplifier, FC the heterodyning stage, IFA the intermediate frequencyamplifier and DET the low frequency detector stage. The oscillationgenerator is the same as that shown in Fig. 2 and like parts in bothfigures are given the same references. ated oscillation ,f at anyfrequency to which it is set is effected by the discriminator D and thevoltage depending uponthe frequency drift of the received frequency isgenerated by discriminator Dl. These two voltages are applied in seriesto the phase control tube PC. The time tor DI and the frequency controltube PC is not fixed. In series with the automatic frequency control isintroduced a four-element bridge rectifier R with capacity C across theline. A battery B is connected in series with the secondary winding of atransformer T, across the free corners of the bridge, in such sense thatthe four rectifier elements of the bridge are made nonconducting. Inthis condition, the time constant of RC circuit is infinity, or at leastvery arge.

A portion of the output from the intermediate frequency amplifier IFA isnow introduced to the other or primary winding of transformer T, and, ifit is sufficient, overcomes the bias on the rectifiers due to thebattery, and passes rectified current through the battery, and throughthe rectifiers. In this condition,'the rectifier impedance may bereduced to a low value, the series impedance along the automaticfrequency control line L is reduced, and the time constant of the linebecomes short. This variable time constant feature forms the subject ofmy copending application Serial No. 449,293, filed July 1, 1942.

In the presence of a signal, the automatic control of frequency isoperative and quick. If, now, the signal is interrupted, or quicklyfades, the time constant of the RC circuit becomes very large, and thefrequency control voltage across the condenser C is maintained.

The two discriminator voltages are connected in series-aiding beforeapplication to the variable time constant circuit R0. The filter F2 atfrequency f is a band-pass filter covering the total range of frequencycontrol required, and this filter itself provides the delay as stated inreference to Fig. 2.

Whilst a particular embodiment of the inven- The stabilisation of thelocally generwill occur to those skilled in the art and which fallwithin the scope of the appended claims.

What is claimed is:

1. A frequency stabilized oscillation generating system comprising aclosed oscillating circuit, an oscillator, an oscillator output circuit,means for shifting the phase of the oscillator output in said outputcircuit, means for introducing said phase shifted output into the closedcircuit, the closed circuit oscillating frequency being varied inproportion to said phase shift, and means for varying said phase shiftin proportion to variations in frequency in the closed circuit andthereby stabilizing said frequency.

2. A system as set forth in claim 1, in which said closed circuitincludes a first modulating means for introducing said phase-shiftedoutput into the circuit, filter means for selecting from the firstmodulating means output a frequency combining the oscillator frequencyand another frequency to which the oscillating circuit is tuned, asecond modulating means for combining the filter output and theoscillator output, and delay means including a filter for supplying saidother frequency from the second modulating means to the first modulatingmeans.

3. A system as set forth in claim 1, in which the means for varying thephase shift comprises a discriminator connected between the closedcircuit and phase shifting means.

4. In a superheterodyne receiving system which includes a heterodyningstage followed by an intermediate frequency amplifier, a frequencystabilized oscillation generating system comprising a closed oscillatingcircuit, an oscillator, an oscillator output circuit, means for shiftingthe phase of the oscillator output in said output circuit, means forintroducing said phase-shifted output into the closed circuit, theclosed circuit frequency being varied in proportion to said phase shift,means for varying said phase shift in proportion to variations infrequency in the closed circuit and thereby stabilizing said frequency,and means for supplying oscillations from the closed circuit to theheterodyning stage.

5. A system as set forth in claim 4, including means for varying saidphase shift in accordance with variations in the output frequency of theintermediate frequency amplifier and thereby compensating for saidvariations.

6. A system as set forth in claim 4, including discriminator meansconnected between the intermediate frequency amplifier output and thephase shifting means, arranged to correct variations in said amplifieroutput frequency by varying the closed circuit frequency throughproportional changes in said phase shift.

'7. A system as set forth in claim 4, including discriminator meansconnected to said amplifier output and means for continuing theapplication of a regulating potential from the discriminator means tothe phase shifting means after the signal producing said potential hasfailed, comprising a capacitance arranged in circuit between thediscriminator means and the phase shifting means to transmit to thelatter a regulating potential from the discriminator means, and means incircuit between the condenser and the discriminator means actuated byfailure of said signal to block discharge of the condenser toward thediscriminator means.

CHARLES WILLIAM EARP.

